Heteroazeotropic mixtures

Nitromethane shows the simplest residue curve map with one unstable curved separatrix dividing the triangle in two basic distillation regions. Methanol and acetonitrile give rise two binary azeotropic mixtures and three distillation regions that are bounded by two unstable curved separatrices. Water shows the most complicated residue curve maps, due to the presence of a ternary azeotrope and a miscibility gap with both the n-hexane and the ethyl acetate component. In all four cases, the heteroazeotrope (binary or ternary) has the lowest boiling temperature of the system. As it can be seen in Table 3, all entrainers except water provide the n-hexane-rich phase Zw as distillate product with a purity better than 0.91. Water is not a desirable entrainer because of the existence of ternary azeotrope whose n-hexane-rich phase has a water purity much lower (0.70). Considering in Table 3 the split... 

Simulation of the separation of the mixture fi-hexane-ethyl acetate by heteroazeotropic batch distillation using acetonitrile... 

S. Skouras, S. Skogestad, Separation of ternary heteroazeotropic mixtures in a closed multivessel batch distillation-decanter hybrid, Chem. Eng. Proc. 43 (2004) 291-304. 

For heterogeneous batch distillation a new double column configuration operating in closed system is suggested. This configuration is investigated by feasibility studies based on the assumption of maximal separation and is compared with the traditional batch rectifier. The calculations are performed for a binary (n-butanol - water) and for a ternary heteroazeotropic mixture (isopropanol - water + benzene as entrainer). Keywords heteroazeotrope, batch distillation, feasibility studies. 

If components of a mixture form a heteroazeotrope or by the addition of an entrainer (E) a heteroazeotrope can be formed, the azeotropic composition can be crossed by decantation. In the pharmaceutical and fine chemical industries batch processes including the batch heteroazeotropic distillation (BHD) are widely applied. As far as we know the BHD was exclusively applied in the industry in batch rectifiers (equipped with a decanter) in open operation mode (with continuous top product withdrawal). The batch rectifier (BR) was investigated with variable decanter holdup by Rodriguez-Donis et al. (2002) and with continuous entrainer feeding by Modla et al. (2001, 2003) and Rodriguez-Donis et al. (2003), respectively. Recently the BHD was extensively studied for the BR and multivessel columns by Skouras et al. (2005a,b). 

Calculations are performed for a binary (n-butanol - water) and for a ternary heteroazeotropic mixture (isopropanol - water + benzene). 

Separation of the ternary mixture (isopropanol (A) - water (B) + benzene (E)) Binary azeotropic charge ( xbaz = [0.674,0.326,0]) is separated by the application of an entrainer ( ). The composition of the ternary IPA - water - benzene heteroazeotrope and those of its it-rich and 5-rich phases ... 

The constant vapor pressure (boiling temperature) above the two-phase region of certain partially miscible mixtures is usually larger (smaller) than the vapor pressure (boiling temperature) at any other liquid-phase composition in the homogeneous region. In this case, the vapor-phase composition is inside the miscibility gap. Mixtures of this type are called heteroazeotropic mixtures, or simply heteroazeotropes. (Fig. 1, Type II), as opposed to the other types of azeotropes, called homoazeotropes. 

Should it prove impossible to effect a satisfactory separation by changing the distillation pressure, the next step will be to find a suitable additive with which one of the constituents forms a.heteroazeotrope, or a homoazeotrope that is easily split up [34]. An approximate method for separating heteroazeotropic mixtures based on the mathematical model for the liquid-liquid-vapour equilibrium of two multicomponent systems was elaborated by BrU et al. [49a]. 

The mode of operation and the dimensioning of a heteroazeotropic distillation as exemplified by the separation of the system water-acetic acid has been described by Wolf et al. [61b]. Morozova and Platonov [61c] analyzed the structure of phase diagrams of multicomponent mixtures using a digital computer. They studied the requirements for the separation of azeotropic mixtures. In order to achieve optimum column combinations Serafimov et al. [58 c] studied the ternary mi.xture isopropanol/ benzene/water on the basis of a mathematical treatment of the separation of heteroazeotropic mixtures. In another paper [58 d] a procedure was presented for the separation into its components of the water-containing mixture with acetone, ethanol, benzene and butyl acetate by means of the thermodynamic and topological analysis of the phase diagram structure. 

In azeotropic distillation, mitures that are difficult to separate are separated by addition of a substance that forms an azeotrope with only one of the components of the mixture. This process is rarely used. A widely used special case is heteroazeotropic distillation, in which water is present as azeotrope component and forms a miscibility gap. 

Mixtures with miscibihty gaps in the liquid phase veiy often - but not always -form azeotropes within the miscibility gap. Since miscibility gaps decrease the boihng point of mixtures (see Chap. 5), such heteroazeotropes are always low boiling azeotropes (minimum azeotropes). 

Fig. 11.3-1 Process for fractionating binary mixtures with heteroazeotrope...

Processes for completely fractionating binary mixtures with heteroazeotropes consist of two distillation columns and one decanter (Fig. 11.3-1). As the azeotrope lies within the miscibility gap of the liqnid the azeotrope can be broken by decantation. The two fractions from the decanter are at different sides of the azeotropic point. Purification of these two rather impnre fractions is performed by distillation. The pure products are recovered as bottoms from the distillation columns C-1 and C-2. 

The process shown in Fig. 11.3-1 utilizes the specific advantages of distillation and decantatioa The advantage of distillation is the ability to produce pure fractions. However, distillation cannot break azeotropes. On the other hand, decantation can break heteroazeotropes but cannot produce pure products. Thus, the combination of distillation and decantation is a very effective process for fractionating mixtures with heteroazeotropes. Such processes are extensively used in industiy. Table 11.3-1 lists some important binary mixtures fiactioiated by the process shown in Fig. 11.3-1. 

The combination of distillation and decantation can also be applied to multicomponent mixrnres. A process for the separation of the temaiy mixture acetone/water/1-butanol is depicted in Fig. 11.3-2. One organic compound (acetone) is miscible with water, the other one (1-butanol) is inmiscible. The mixture water/l-butanol exhibits a heteroazeotrope. A boundary distillation line runs from the heteroazeotrope to the low boiler acetone. 

P11.4 In the free DDBSP Explorer Version, search for all mixture data for the system benzene-water. Due to very limited miscibility in the liquid phase, the system shows heteroazeotropic behavior. Estimate the azeotropic composition using the vapor pressure correlations given in Appendix A and compare the results to the experimental data. 

To understand the mutual behavior of the components depending on the degree of the mixture s nonideahty caused by the difference in the components molecular properties, it is better to use graphs yi - x, T - xi,T - yi,Ki - xi, and K2 - xi (Fig. 1.2). In Fig. 1.2, the degree of nonideality increases from atoh.a is an ideal mixture, h is a nonideal mixture with an inflection on the curve y - xi (a and b are zeotropic mixtures), c is a mixture with a so-caUed tangential azeotrope (curve yi - xi touches the diagonal in the point xi = 1), d is an azeotropic mixture with minimum temperature, c is a mixture with a so-called inner tangential azeotrope, / is a mixture with two azeotropes, g is a heteroazeotropic mixture, and h is an azeotropic mixture with two liquid phases. Azeotrope is a binary or multicomponent mixture composition for which the values of phase equihbrium coefficients for all components are equal to one ... 

Heteroazeotrope is an overall composition of a mixture with two liquid phases for which the values of the overall coefficients of phase equihbrium for all components are equal to one ... 

Figure 1.2. Phase equilibrium of binary mixtures (a) ideal mixture (b) nonideal mixture (c) tangential azeotropic mixture (xi. Az = 1) (d) azeotropic mixture (e) mixture with internal tangential azeotrope (0 < xi, Az < 1) (f) urixture with two azeotropes Azi and Az2, (g) heteroazeotropic mixture and (h) azeotropic mixture with two liquid phases (y — x, T — x — y, and K — x diagrams). Az, azeotropic or heteroazeotropic point x i and x 2, compositions of liquid phases.

Further increase in nonideality and transition to heteroazeotropes makes it again possible to separate mixtures, not using just a distillation column, but a column with decanter complex. Cases e and / occur, but very seldom therefore, we will not consider them further. 

In real mixtures, functions Ki j have rather complicated form (especially for azeotropic and heteroazeotropic mixtures). 

Separation of Azeotropic Mixtures by Distillation Under Two Pressures or Heteroazeotropic and Extractive Distillation... 

On many occasions, general geometric theory of distillation allows development of flowsheets of multicomponent azeotropic mixture separation without using such special methods as distillation under two pressures or heteroazeotropic and extractive distillation with entrainers (i.e., with additional components injected into the unit). 

Figure 2.13. (a) A heteroazeotropic column with decanter, (b) the distillation trajectory of heteroazeotropic column for separation of benzene(l)-isopropil alcohol(2)-water(3) mixture (benzene-entrainer). xli and xl2, two liquid phases xp, initial feed xp+p, total feed into column region of two liquid phases is shaded. 

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